PTF1 J085713+331843, a new post-common-envelope binary in the orbital period gap of cataclysmic variables

Abstract

We report the discovery and analysis of PTF1 J085713+331843, a new eclipsing postcommon- envelope detached white-dwarf-red-dwarf binary with a 2.5 h orbital period discovered by the Palomar Transient Factory. ULTRACAM multicolour photometry over multiple orbital periods reveals a light curve with a deep flat-bottomed primary eclipse and a strong reflection effect. Phase-resolved spectroscopy shows broad Balmer absorption lines from the DA white dwarf and phase-dependent Balmer emission lines originating on the irradiated side of the red dwarf. The temperature of the DA white dwarf is TWD = 25 700 ± 400K and the spectral type of the red dwarf is M3-M5. A combined modelling of the light curve and the radial velocity variations results in a white dwarf mass of MWD = 0.61+0.18-0.17M⊙ and radius of RWD = 0.0175+0.0012-0.0011 R⊙, and a red dwarf mass and radius of MRD = 0.19+0.10-0.08M⊙ and RRD = 0.24+0.04-0.04 R⊙. The system is either a detached cataclysmic variable or has emerged like from the common-envelope phase at nearly its current orbital period. In ~70 Myr, this system will become a cataclysmic variable in the period gap.